Electric-wave secrecy transmission



C. A. SPRAGUE ELECTRIC WAVE SECRECY TRANSMISSION Filed Nov. '17. 1922 Sep t. 1,1925. I 1,551,707

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C/arewceA Spray:

Patented Sept. 1, 1925.

UNITED STATES q 1,551,707 PATENT, OFFICE.

CLARENCE A. SPRAGUE, OF EAST ORANGE, NEW JERSEY, ASSIGNOR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

Application filed November 17, 1922. Serial No. 601,471.,

To all w/zmn it may concevm:

Be it known that I, CLARENGE A. SPnAGUE, a citizen "of the United States, residing at- .East'Orange, inthe county of Essex, State of New Jersey, have invented certain new and useful Improvements in Electric-\Vave Secrecy Transmission, of which the following is a full, clear," concise, and exact description.

-The present invention relates to electric wave transmission for-signaling or kindred purposes, and more particularly to secret transmission of signals and the like.

The invention is concerned withthe' type of signal transmission-in which a sustained wave, hereinafter termed a carrier wave, is modified in some manner by the indications to be sent. It has been proposed to render a system of this general type secret by causing the carrier freq'uency to vary cyclically and in an erratic manner so that one not knowing the law of variation of the carrier frequency would have great difficulty in receiving the signal indicationsin an understandable form. Such a system is disclosed, for example, in Canadian Patent 197,181, issued February 10, 1920.

Varying the carrier frequency increases the frequency range employed in the transmission of the message over the range. that would be required if the carrier frequency remained constant. This increased frequency range required for an individual message becomes of considerable importance particularly 'in the case of multiplex systems in which a different frequency rangeis set aside for use exclusively in transmitting each separate message. L

It is an object of the present invention to provide multiplex secret transmission by carrier waves of varying frequency in which the range employed for one signal overlaps 'that employed for another so that the total 'rangeof frequencies used for three simultaneous messages, for example, is less than three times the range used for a single message. I

A feature of the invention is the-use of a plu -ality of carrier waves, the frequencies of w iich vary together preferably in a continually and definitely related manner.

A furtherfeatiure of the invention is the I use of a plurality of carrier waves,'the frequencies of which vary within substantially the same extreme limits and in which the ELECTRIC-WAVE sEcREcY TRANSMISSION.

waves, the frequencies of which may vary over the same range.

Other objects and features will be apparent from the more detailed description to follow.

In accordance with the specific embodiment of the invention to be described, a number of carrier waves quencies determined from a common control device in such a way that each wave follows through substantially the same course of variation, but the frequency limits have their frebetween which any one wave varies overlap i the limits between which another wave varles. In the extreme case to be specifically described, in which aplurality of carrier waves vary in frequency between the same limiting frequencies, the carrier wave frequencies execute different. portions of'th'eir cycle of variations at any one time. That is,

the cyclic variations of the respective car? rier frequencies are displaced in phase. Each 'CflIllBI' wave is modified in accordance with 'a different signal, such as a different conversation, and the waves are neously transmltted. I

To receive the messages so transmitted, a

simultaplurality of locally controlled waves may be employed, the frequencies of which vary cyclically 1n 'the'same manner as the frequencies of the carriers to be received.

'These locallyv generated waves may each vary about a different frequency so that at any instant the'difference in frequency between one such locally produced wave and one of' the received carriers has one constant value, While the difference in,frequency between another locally produced wave and one of the received carriers has another constant value, and so on. However, if desired,

the locally produced waves may vary about the same frequency. In either case, the difference in frequency between one and only one of the locally controlled waves and a given carrier has a constant value since only in the case of this one of the locally produced waves does the frequency variation agree with the frequency" variation of the particular received carrier in question. A differentlocally produced wave has a constant frequency difference from a different received carrier wave, and so on. This forms a basis of separation between the received waves. The received wave complex is combined in one circuit with a single one of the locally produced waves and a narrow band of frequencies including the constant difference frequency that results from interaction between this local wave and the particular received wave with which its frequency variations are in phase agreement is selected foi' separate detection. In another-circuit the received wave complex is combined with another of the locally produced Waves and the resulting constant difference-frequency wave with itsmodification, representing a different message, is selected into a different receiving circuit. By this process the individual messages are separately received. F

N ot only is the frequency range required for a given plurality of messages in this manner verymaterially reduced over that which would be required in systems hereto fore suggested, but the degree of secrecy is increased due to the materially greater difficulty in receiving a message without a knowledge of the frequency relations employed. i

In the drawing, a terminal of a two-way signaling system embodying the invention is' diagrammatically illustrated.

In this drawing three two-way channels of communication are shown arranged for operating over the samehigh frequency carrierlwave transmission path. This path is indicated as comprising in parta grounded conductor 1 which may be continued" in the form of a transmission line to a distant station in case the carrier waves are to be guided to a definite point, or which may lead to an antenna in case the carrier waves are to be radiated to the distant point or points. This conductor serves in both transmitting and receiving carrier waves. The transmission circuit 1 is provided with a balancing network HN and a differential repeating coil or hybrid coil H in accordance with well-known repeater ractice for enabling waves to be received rom the circuit 1 and impressed on it without mutual interference. The network HN may contain a number and arrangement of electrical elements sufi'icient to simulate, as a whole, the circuit lying to the other side of the hybrid coil H, as will be apparent to those skilled in the art. Between the transmission cir, cuit l and the hybrid coil is included a broad band filter HF for confining the frequencies transmitted between the conductor 1 and the terminal circuits to a definite range of frequencies. This filter, as well as receiving carrier wave channels may be connected to the receivin g loop RL, and similarly any number of transmitting carrier channels may be connected to the loop TL.

Two of the channels comprising the respective transmitting sides T and T and the receiving side R andR are shown provided with low frequency terminations in the form of telephone lines L and L respectively. Each of these lines is provided with a balancing network N or N and a differential repeating coil to which the transmitting and receiving sides of the respective channel are differentially related. For the sake of illustration, the third channel is shown with a different low frequency termination, its transmitting side T leading directly from the microphone 10 and its receiving side R leading. directly to the telephone receiver 11.

The circuit arrangement of each of the channels T T and T may be the same,

exceptfor their frequency relations and through an amplifier A,. This modulator is shown as comprising a. thermionic discharge tube of Well-known type, although any suitable type of modulator. or modulator circuit might be employed. The oscillator 0 is of the general type disclosed in U. S. patent to Hartley 1,356,763, October 26, 1920, except that this oscillator is provided with a condenser 12 arranged to have its capacity continuously varied-in a manner to" be described later. The continual variation of capacity of condenser 12 causes the frequency of Zthe wave generated bythe source 0 to be continuously varied. This varying frequency carrier wave is supplied through the coupling 13 to the grid circuit of the modulator M the condenser 14.- form- Tu 1,561,707- I ing a. path (if low impedancefor v 4 rier -frequency'iinxshunt of the'voice fre lqduency input'Icoil' connected t-ol the line The receiving'sid'e R, of this channel 1),, and low pass filter LF the outputside of which is connected to the line L,. For

a better understanding of the significance as these terms are employed in this descrip- -'tion, reference is made to the April 1921 ony and telegraphy, by Messrs. Colpitts number of the Journal of the American Institute of Electrical. Engineers, page 311,-in

the article entitled Carrier current teleph:

and Blackwell. Also associated with the receiving circuit R, are the. oscillatorslI-IO and 16, the functions of which willbe "described'later. The filter F is designed to have a transmission frequency range broad.

enough to accommodate all ofthefrequem 'cies; comprised in the waveto be received by.

this channel and, may be d'esigned in accordance with the disclosure of the, Campbell patent referred :to.- The demodulatorDM not be thesame.- It is provided with a condenser 17 adapted to. have its capacity continuously varied so thatthe wave which is generated has a continuously varying" frequency; The filter RF isdesigned to se-' lectivelyr transmit to the detector D the particular components of the wave in the out vput of the. demodulator circuit which contain the signal indications 'to be detected. I This filter wouldordinarily "be designed to. I have alrelativelynarrowtransmission range. ,The detector D into'which' the filter circuit RF feeds, is shown as comprising an electron discharge device, of 'the audion :type.

" .A n oscillator v16- ofgconstant frequency is I preferably associated withthis detector, 211- though, .in-general, this oscillator-'is not I 11!. ferred to. I

necessary. This-oscillator, may be; 'of the type disclosed in r the Hartley patent re- As stated,

with the channel T J The carrier oscillator 0 is provided with 'a condenser-"1'9 and the ing frequencies; Also the heterodyne ioscil-- lators I and" H0; have the respective variable condensers 21 -and22 for causing the heterodyning frequency to vary continte circuit elements f' of the. channels T '{andT; may be duplicates of those showngf 'and. described in connection I uously. The Econdensers 12-, 19 and 2 for controlling thefrequencies. of the respective carrier waves are' caused to vary in a de fi (nitely related ,rna'nner, as, for example; by having their movable elements connected 26. In asirnilar manner the condensers ll'l, 21 and 22 have theirimovable elementsfixed' to a common shaft 27 driven fIOmSFLtILlQt-QI 28i- While separate 'motors for the -two shafts have been shown, it would be obvious to drive bot-h shafts from a single motor, I

Any suitable manner of varying the-ca; p'acities of these condensers may be-"enr, ployed, as by simply rotating the movable element, as in an ordinary air-condenser,.or by rotating the movable element, and in ad-' v dition making the movable plates withs'callops or some, kind of irregular configuration to give an irregularvariationto the 'c'apacity. "VJ Y 1. As explained in (conhectioniwith theifilter' F,, the filters F and -F aredesign'ed'to have transmission ranges corresponding to the frequencieskof the waves to be received by .these w.channels.f-- In the specific case jiin which all of the carriers-used for transmits ting in one direction vary within the same frequency limits, the filters F I landF 'would have the same transmission-ran e. In such a-case the filters/E 'F 'andcould be omitted fromi the respective re-:

ceiving channels', .flandif desiredfa' single filter, the equivalent of oneof thein could be included inv the branch RW between the hybrid coil H and the, receiving fchannels'; And if the frequencies of the carriersused for transmitting in both directions -va1=y: within the same frequency limits,-thefilters. F F and E would? each have ethersame; transmission ran euas the filter HFLsoEthat the one filter .H wouldsufiice., -::However,

the frequenciesnsed for transm-ittingin one direction-may comprise az difl'erentrange; 3

from those used for transmitting-in the opposite direction, and likewise the invention isgnot limited tothe casein which the frequencies used for transmitting; in the same direction vary within thesameifrequency limits. a

The operation is as follows: Theinotor'; 26 drives-the condensers ,12,-19 ,a1 d; 201:0" cause the frequencies fofrthe carrilergwa used by the three'i'transrnitting channels to vary in a definitely related-manner so ithatx the total range of frequencies usedbyfthe- Channel T for example, overlaps the total v 1 range of the, frequencies used by, the channel T,i,.and",the totaliranggz ,used by the.

channel T .oVerla-ps the total; rangewuse'd..

bylthe channel-Th [By definitely relating I the: frequencyvariations the carrier- JWBIYQ of, each" channel to those of the adjacent channel, however, the messages of thesev-' eral' channels are transmitted without in V i to the sameshaft 25 drivemfrom the motor in the oscillatorsthat the variations in the frequencies of the respective carrier waves are displaced in phase by a suitable amount.

Speech or other signals received over the line L pass through the coil .15 to-the modulator circuit M which is also supplied with the carrier wave from the 0scillator 0,. By the well-known action of the modulator circuit, the carrier wave is modulated in accordance with the signal wave and the resulting modulated wave is amplified at A and impressed on circuit TL. In a similar manner, signals received from the line L modulate at M the carrier wave impressed from the oscillator 0 and the wave so modulated is amplified at A and impressed on the same circuit TL. This circuit TL also carries the wave received from the channel T comprising a modulated wave produced at M from interaction ofthe carrier wave from the source 0 with speech waves spoken into the Micro phone 10.' This complex of waves in the 1 loop TL is impressed on the hybrid coil H and is transmitted through the broad band filter HF to the transmission circuit 1.

The distant station to be communicate 'with may have an' arrangement entirely similar to that shown in the drawing, and

"- the manner in which the signals transmitted from the station shown are received at the distant station will be clear from considering the action of the circuit shown in the drawing in receiving signals.

The complex of waves incoming on the conductor 1, and similar to the complex of waves described above as being transmitted, pass through the filter HF, hybrid coil H (v and into the receiving loop RL. In the case assumed in which each of the received car -riers varies over the same variation range,

all three of the received carriers, together with their signal modulations, will pass through the filters F F and F In fact in this case these filters, are not essential but might be omitted. The local sources HO,, H0 and Ho;01- producing the auxiliary waves 'usedin receiving, have their frequencies varied according to the same law of variation as'the received'carrier waves. This may be done by constructing the condensers 17, 21' and 22 in a similar manner to the condensers used in the transmitting oscillators at the distant station. and in driving the respective condensers atthe two staner to run in synchronism "with the motor 28. In'the simpler case in which a single motor is used at each station in place of the two moto'rs shown, any known system, of Which there are several, for synchronizing these single motors at the several stations may be employed. I

By causing the variations of the wave from the source H0 for example, to follow those of one ofthe received carrier waves and to be equal in amount to the variations in frequency of the received carrier wave, the beat or difierence frequency Wave obtained in the output of the demodulator DM is of constant frequency, except, of course, for such effects on the frequency as are caused by the variations representing the signal. The actual frequency of this beat wave may be made any desired value depending on the frequency of the local source H0 The filter RE is designed to select this beat frequency wave from the other output components of the demodulator and to supply it to the detector D Asimple detector might be employed at this point to yield the signal indications from the heat wave. However, a homodyne or zero-beat source 16 is shown, this source having the same frequency as the wave frequency of the beat wave, independent of the variations in this beat wave representing the signal itself. I

One advantage of employing'such' a zero beat source is that it aids materially in selecting the desired component from the un desired components since it has the effect" of selectively amplifying the wave with which it agrees in frequency, that is, the wave carrying the signal indications to be received. After detection, the. signal components, such as speech currents in the-case.

of telephony,- are selectively transmitted L through the speech filter LF to the line It will be seen that only one ofthe received carrier waves. will produce a beat wave of constant frequency by interaction with the source 'HO At nearly all times the beat wave between either of the other received car rier waves and the auxiliary wave from HO will have a different frequency fromthat of the constant heat wave 1 which it is desired to receive in this channel, and hence the filter RF will not select the other beat waves, which it will 'be noted, vary in a still more erratic manner than the received waves onaccount of the extra variation factor introduced by source H0 There will be instantaneous times in each cycle of variation when-the .wave' to be received in this channel agrees in frequency v with another one of the waves, but by aramount because of the ezrtremely short interval during wlnch a neighboring channel cuts in.

The generator HO of the second channel is. controlled as described so that its frequency variations follow those of a different one of the received carrier waves and the beatwave ofconstant frequency that is passed through the filter R1 therefor carries signal indications of a different message from those of the wave received in the channel R Similarly HO supplies a wave which" whencombined with the received waves produces a beat wave of constant fre ,quency with .a still different one of the re- .ceived waves.

It is preferred that the frequencies about which the waves of the respectivesources H0 H0 and H0 vary in frequency be different so that the beat wave in the first channel has one constant frequency while that in each of the others has a different contransmission to a distant point.

The foregoing description sets forth the preferred arrangements for practicing the invention. The scope ofthe mvention will now be defined in the appended claims. What is-claimed is:

- '1. In a carrier wave transmission system a common transmission path. means to generate and transmit over said common 'path a plurality of different carrier waves and commonly-controlled means for continuously varying the frequency of each of said waves by the same amount.

2. In a carrier wave transmission system, means for generating and transmitting a plurality of carrier waves of different frequencies and a common controlling means for causingthe frequency of each of said waves to undergo periodic variations of the same magnitude.

3. In a carrier wave transmission system, means for generating and transmitting a plurality of different carrier waves and a common controlling'means for causing the frequency of each of said waves to undergo characteristic periodic variations of the same magnitude.

i 4 In a multiplex-carrier wave transmission system, a plurality .of wave generators for producing each a different carrier wave,

means associated with each of said genera- 5. In a. carrier wave transmission system,

,-means for generating and transmitting a plurality of different carrier waves, means.

for causing the frequency of each of said waves to undergojsimilar periodic variations within a prescribed frequency range, and means for coordinating the variatlons of the respective carrier wave frequencies I so that each of two carrier waves occupy at different instants a portion of the prescribed frequency range of the other.

6. In a carrier wave transmission system, means for transmitting a plurality of carrier waves, means for cyclically varying the frequency of each of said waves within definite extreme limits, the range throughout which each of said carrier waves varies in frequency over-lapping the range throughout which another of said carrier waves varies.

7. In a carrier wave transmission system,. means for generating. and transmitting a plurality of carrier waves and for causing each of said waves to undergo a periodic variation in frequency and means for coqrdinating the variations in frequency of the respective waves so that the frequencies of a plurality of said waves pass successively through a common range.

8: In a carrier wave transmission system, means for generating and transmitting a plurality of carrier waves and for causing each of said waves to vary in frequency periodically throughout the sanfe range of frequencies, the frequency variations of the respective carrier waves being displaced in phase relativeto one another.

9. In a carrier wave transmission system means for generating and transmitting a plurality of carrier waves and for causing each of said waves to vary in frequency periodically between the same frequency limits, and means for coordinating the variations in frequency of said waves whereby different ones of said waves occupy at'different times the same portions of the frequency rangecomprised between said limits. I

10. A system according to claim 1 and including means for impressing signal indications representing a different message on each of said carrier waves.

11. In a carrier wave signaling system, means to transmit a plurality of carrier waves and to impress a different signal on each of said waves, means to vary the fre quency of each of said waves cyclically throughout a definite frequency range, the range of variation of each of said carrier waves over-lapping the range of Variation of another. of said waves, a common receiving circuit for said waves, individual re ceivers for said different signals, and means for separating each of said different signals to a-ditferent receiver.

12. In a signaling system means to transmit aplurality of carrier waves and to vary the frequency of each of said waves by the same amount in a characteristic manner, means to receive'said carrier waves, a plurality of sources of auxiliary waves, and means to vary the frequency of each of said auxiliary waves in the same characteristic manner as one of the transmitted waves. "13. In a signaling system employing a signal-controlled carrier wave of constantly varying carrier frequency, means to receive the *slgnal, comprising a demodulator, a local source of sustained waves of constantly varying frequency, the frequency of the locally generated waves being from instant to instant different from the frequency of the carrier wave by substantially a constant and appreciable amount, connections for impressing both the received carrier wave and the locally produced wave upon said demodulator, and a signal receiver associated with the out-put of said demodulator.

14. In a signaling system, means to trans- -m1t a plurality of carrierwa'ves and to impress a different signal on each of said waves, means to cause each of said waves to undergo characteristic periodic variations of the same magmtude 1n frequency, means \to receive said waves, a plurality of sources of auxiliary waves, means to vary the frequency of each of said auxlllary waves in the same characteristic manner as one of said carrier waves, and means to cause interaction between each auxiliary wave and the carrier wave havmg the same characterlst-lc frequency variation, to receive the respective signals impressed on said carrier waves.

15. In a signaling system, means to transmit a plurality of carrier waves and to impress a different signal on each of said waves, means to cause each of said carrier waves to undergo characteristic periodic variations in frequency in such manner that the respective frequency ranges throughout which said waves vary overlap each other, means to receive said carrier waves, means to step down the frequency of each of the received waves by a variable amount corresponding to the variable frequency of the respective wave to produce resultant waves ofconstant frequency each of which has impressed upon 1t one of said signals, and means to derive the signals from the respective waves of constant frequency,

16. In a signaling system, means to transmit carrier waves of constantly varying frequency, the frequency of each Wave varying in a different characteristic manner, means to impress a different signal oneach wave, a common receiving circuit for all of said waves, as many demodulators as there are carrier waves, a source of heterodyne waves of constantly varying frequencyassociated with eachone of said demodulators, each of said sources having a different characteristic manner of frequency variation in accordance with the characteristic manner of variation of one of said received carrier waves, means to impress all of the received waves on each of said demodulators to cause interaction between all of said waves and each of said heterodyne sources separately, a circuit associated with the output of each "demodulator for selecting out the constant respectively similar frequency-variations to produce difference frequency waves of constafit frequency each controlled by a signal, and means to derive the signals from the respective waves of constant frequency.

18. In a signaling system employing variable frequency. carrier waves, a common receiving circuit for said waves, as many branches to said circuit as there are waves to be received, means in each branch for differently changing the frequency of the re ceived waves by a variable amount to pro duce from'one of the received waves in each branch a resultant wave of constant frequency and from all the other received waves resultant waves of variable frequenl cies, a selective circuit connected to each frequency changing means for selecting the wave ofconstant frequency, and signal receivers connected to the respective selective circuits.

19. In a signaling system means to transmit a carrier wave of cyclically varying carrier frequency and to impress sound wave variations on said wave for transmission, means to receive said carrier wave of vary' ing frequency with its sound wave variations and to produce therefrom a carrier wave ofconstant frequency with said sound wave variations impressed upon it.

20. In a signaling system means to transmit a plurality of carrier waves each of cyclically varying carrier frequency and to impress difli'erent signal indications on the respective waves, and means to receive said waves and to convert the received waves of variable frequency into waves ofconstant and respectively different frequency each having impressed on it the same signal indications as the carrier wave from which it is converted.

21. In a signaling system means to transmit a plurality of carrier waves. each of cyclically varying carrier frequency andto respective waves for transmission, means to receive said waves and to produce from them respective waves of constant and re-' spectively different frequency each having the same signal indicationsimpressed on it .as the corresponding received .wave, means to select each of the wave's of constant frequency and means tofdetect the signal indications of the selected waves.

22. In-a signaling system means to receive a carrier wave of continuously ovarying carrier frequency, a local source of o-s-- cillations of frequency varying continuously in similar manner to the recelv'ed waves, a modulatingcircuit, means to impress the received waves and locally generated oscillations on said circuit, a filter for selectively receiving from said circuit the wave component of frequency equal to the difference between the frequencies of the impressed waves, a detector, a circuit-for impressing the selected wave component on said detector, and a signal receiver connect- -edto said detector. v e

23. In carrier transmission, the mefiiod comprising transmitting a" plurality of. carrier waves over a common path and continuously varying the frequency of each of said waves by the same amount in definitely r'elated manner to secure secrecy of transmission. I

- 4. The method of secret signaling'comprising modulating each of-a plurality of.

carrier waves by a different signal, continu- 'ously varying the frequencies of each of said said waves.

' carrier waves by the same amount in related manner and simultaneously transmitting impress different signal indications on the prising'transmitting a plurality of carrier waves, varying the frequency of each of said waves cyclically by the same amount in definitely related manner, controlling each of the transmitted waves in accordancewith a different signal, receiving said waves and, separately detecting the signals transmitted by each wave.

26, The method of secret signaling comprising transmitting a plurality of carrier waves, varying the frequency of said waves cyclically, controllingeach of thewaves in accordance with a different signal to be transmitted, receiving said ing at the receiving point ocal vwaves of cyclically varying frequency, combining each of said local Waves with all of there- 25. The method ofsecret signaling com-' aves, producceived waves to produce difference-frequency L waves, selecting the 'waveshaving a conto derive the signals.

27. The method of secret signaling comprising transmitting a plurality of carrier waves, varying the frequency of said waves cyclically, controlling each of the waves in.

accordance with a different signal to be transmitted, receiving the waves, producing at a receiving point local waves of cyclically varying frequency, the frequency of said local waves varying in unison each with the frequency of a different one of the received flo waves, combining each of said local waves with the received waves toproduce asmany difference-frequency waves of constant frequency-as there are local waves, and selecting and detecting each of said differencefrequency waves to reproduce the signals.

28. A system according to claim 6 and including means-"for impressing signal indications representing a different message on each of said carrier waves.

In witness whereof, I hereunto subscribe my name this 11th day of November A. D.,

- CLARENCE A. SPRAGUE. 

